Shannon Ashcraft, Adam Bilodeau, Shiva Ganti and Tim Harris, ENTRUST Solutions Group, USA, highlight the unique considerations at play in the design and construction of renewable natural gas interconnect stations.
Recently, a Liberty Utilities NY (Liberty) company called on ENTRUST Solutions Group (ENTRUST) for engineering design services for a pipeline and interconnect station for a renewable natural gas (RNG) supply at Stauffer Farm in Lawrence, New York, USA. The proposed RNG project aimed to condition biogas, generated from cow manure on the Stauffer Farm site, to pipeline quality prior to injection into the Liberty system.
First in the equipment flow process is the digester, where the raw biogas is created from the feedstock, in this case cow manure, via an anaerobic process.
Then, RNG upgrading occurs. This is performed upstream of ENTRUST’s portion of the project by the RNG developer who is responsible for converting raw biogas into RNG.
Thirdly, the RNG interconnect station takes custody of the RNG from the developer, analyzes it and regulates it before allowing it to enter Liberty’s Natural Gas system. ENTRUST took responsibility for this portion of the process.
Conditioned gas was to be delivered to Liberty’s existing 8-inch steel transmission pipeline via a proposed 2-inch pipeline.
ENTRUST was familiar with the complexities posed by these types of projects from experience working for other gas utility clients across the United States, having successfully designed and assisted in permitting other projects with similar requirements. The engineering design package included civil and mechanical design, electrical design, and structural design.
The project was led by Project Manager, Shannon Ashcraft, based in ENTRUST’s Marlborough, MA office and supported by Project Engineers, Adam Bilodeau and Shiva Ganti. Bilodeau was the engineering lead for mechanical design, while Ganti oversaw the electric and controls portions of the project.
Ashcraft explained, “RNG is a fuel that can be derived from several different sources and is a relatively new concept for many of our gas utility clients. When we engage with clients on an RNG project, typically it is only their first or second experience with the process.”
Since RNG is a relatively new energy source for gas utilities, ENTRUST had to understand and account for any potential concerns Liberty might have had with the RNG interconnect station before any major portion of the design process was underway. That way, fixes to alleviate those concerns could be built into the design from its inception.
Since it was Liberty’s first time working with an RNG interconnect station, ENTRUST engaged with Liberty to determine what they wanted to achieve with the facility and the types of equipment their operations crews preferred.
While the RNG interconnect station was something new and different, ENTRUST wanted to ensure the design was similar to Liberty’s existing facilities, to allow their personnel to more easily maintain and operate the site.
During the preliminary design process, various types of equipment that the utility was familiar with (such as gas chromatograph) were identified.
During the preliminary design process, various types of equipment that Liberty was familiar with (such as gas chromatograph) were identified. Wherever possible, equipment familiar to Liberty was incorporated into the design to reduce any potential complications or equipment learning curves down the road. When these items were determined, a more detailed design phase began, progressing to a more typical process for standard gas facility projects.
RNG can present as a different chemical composition depending on how it is derived (via cow manure, chicken manure, wastewater, etc.) and in turn, each is very different from natural gas in terms of composition and ultimately, quality.
Since the composition of RNG is unpredictable, some degree of interface between the gas utility and the producer is necessary. If that is not possible, investment must be made in analysis equipment, which can be used to check the gas composition to determine whether it is necessary to stop the flow of gas into the pipeline. Consideration must be paid to the downstream effects of that gas on the system.
After a problematic gas composition is identified and addressed, a process needs to be in place for retesting the RNG. Since most natural gas equipment is designed for ‘standard’ natural gas, it must be ensured that it will work correctly with some of the higher sulfur levels or other components encountered in RNG.
Ganti explained, “If the composition of RNG gets out of specification and becomes unacceptable for the pipeline, there is usually an automated valve that stops the gas. At that point, the gas distributor/interconnect owner would go back to the producer and inform them that they had failed to meet the specifications. There is another chance given to retest, but if the gas fails again, it is shut out again. This is the mechanism by which gas quality is governed and enforced. Of course, the producer wants to avoid being shut out because any time they are not sending gas down the pipeline, they are losing money.”
Even though the supplier is responsible for ensuring the gas is at a level of quality that the utility is willing to accept, the distributor also needs to have checks in place to ensure that that gas continues to maintain its quality. The distributor will, therefore, also be measuring the different components in the gas to make sure that it stays within the nominal ranges that are required or expected via code and to ensure that it will also work with their system and not damage any equipment downstream.
ENTRUST regularly undertakes gas transmission station projects, but the unique part of this RNG work was determining the components needed to measure that gas and the processes that should be in place if out of spec gas were to be encountered.
Part of the challenge was to design the gas composition feedback process and determine the type of notifications that should go to the customer to inform them that the gas is out of spec. Then, which processes should happen automatically inside the facility to stop that gas from continuing into the system until the supplier can bring the RNG back into a suitable composition range.
ENTRUST worked with Liberty extensively to determine where everything would fit in the space-constricted site to adhere to New York state’s rigorous equipment safe distance codes.
Bilodeau said, “Managing sufficient spacing of equipment within the confines of the site was a challenge. Challenges with sizing were due to the high pressures the RNG was being delivered at, around 600 PSIG, relative to Liberty’s outlet pressure of 150 PSIG and 60 PSIG. High pressure cuts cause heat loss in the gas that can lead to freezing of equipment.
Normally this would be solved using line heaters to pre-warm the gas prior to the pressure cut. However, given the low flow of the facility, this was not economical. We had to use instrument heaters to avoid freeze-offs on critical components.”
An additional issue was posed by the fact that New York State code requires 50 feet be maintained between overpressure protection devices. However, the site space was limited to a 60 foot by 60 foot space. Since multiple pieces of equipment required 50 feet of spacing, this created challenges with the site layout.
The station supplied gas to Liberty’s system, but also back to the farm. If the digester was shut off, the station would back feed to the farm. This led to a situation where the station could run forwards or backwards. Because the inlet would become the outlet and vice versa depending on the operating conditions, ENTRUST had to design for additional space constraints.
Bilodeau explained, “The digester plant requires natural gas to operate. When operating, the farm can ‘buy back’ the RNG they have produced from Liberty. However, if the plant is shut down and not producing RNG, there will be backfeed through the station. In this case, the station outlet becomes the inlet, and the station must flow in reverse. This triggers additional code requirements, since the station is now bi-directional. Given the limited site spacing, and the requirement for equipment to be spaced for fire safety reasons, we had to make design alterations to ensure we were following the required codes.”
In addition to this project, Liberty was also working on some pressure changes to the pipeline that it was connecting to. Bilodeau said, “Liberty’s initial setup was for an outlet pressure of 150 PSIG but within the next five years they also planned to be operating with a 500 PSIG outlet pressure for another setup. Because of this major anticipated jump, we had to validate equipment selection for different future operating conditions, and size everything twice, so that the equipment would work both in the current conditions and their anticipated conditions five years down the line.”
Tim Harris, Principal Engineer, Hydrogen & Molecular Renewables Emerging Energy Solutions, ENTRUST, said, “Another unique challenge for this project was the modification and guidance of gas composition standards. Liberty had never received dairy-based RNG into their system previously. As such, they only had previously established gas quality specifications. ENTRUST helped Liberty to assess potential risks and constituents of concern associated with dairy-based RNG. Working together, and evaluating the local system flows, suggested RNG constituent thresholds were established. Additionally, a general process of how often to test the RNG source, and how testing frequency should be modulated based on previous results was established. Ultimately, Liberty was able to provide definitive guidance to RNG developers, enable the injection of RNG into their system, and protect their existing customers and system from variations in gas quality.”
There were several factors that helped contribute to the overall success in the project, with the most significant being that the RNG interconnect station was fully constructed in line with Liberty’s tight construction schedule, and it continues to run well.
Another measure of the project’s success is that ENTRUST has been invited by Liberty to bid on additional RNG facility projects with slightly different scopes.
Ashcraft said, “Liberty reported that they enjoyed working with us, they appreciated the product that we gave them and we provided invaluable support specifically in the construction portion, working with the contractors to make sure that everything got set up appropriately with the RTU and the electric elements.”
RNG is a growing space in the industry, with the potential for more than ten times the current RNG supply by 2050.
Many operators are therefore looking to expand into the RNG space, especially as there is a growing drive towards net zero carbon. RNG will play a significant role in helping utilities offset their carbon footprint.
Ashcraft added, “The successful completion of this project has been a proving ground for Liberty, since it was their first, but the successful execution on the design of an RNG interconnect station has also provided ENTRUST with a much deeper understanding of how these projects come together. We are now well positioned to bring that experience into projects with other clients, helping them to navigate through their next RNG facility construction projects. The RNG space is certainly a growing one for ENTRUST.”
Please Sign Up to get Our Latest Newsletter
myENTRUST